The present invention relates generally to electronic circuits, and in particular to input ranges in differential amplifiers.
Many electrical circuits have differential amplifiers for amplifying a difference in voltages between two input signals to produce amplified differential output signals.
FIG. 1 shows a typical differential amplifier 100 having driver transistors N1 and N2, load transistors P1 and P2, and a current source N3. Differential amplifier 100 receives differential signals INA and INB and outputs differential output signals OA and OB.
Differential amplifier 100 usually has a lower limit and an upper limit for the signal levels of the INA and INB signals. These limits specify an input range. Differential amplifier 100 operates properly when the INA and INB signals stay within the input range.
When the INA and INB signals exceed the lower limit, N1 and N2 could turn off. In the opposite, when the INA and INB signals exceed the upper limit, transistors N1 and N2 could operate in linear mode and act as resistors. In either case, differential amplifier 100 operates improperly when the INA and INB signals exceed the input range.
The various embodiments of the present invention provide methods and circuits for increasing the normal input range of differential amplifiers to allow the input signals to exceed the lower and upper limits of the normal input range.
In one aspect, a circuit includes a differential amplifier having a first differential input node and a second amplifier input node for receiving differential input signals, and having an amplifier current source. A current regulator has a pair of input transistors for receiving the differential input signals to regulate a current of the differential amplifier
In another aspect, a circuit includes a differential amplifier having a pair of driver transistors for receiving input signals. A current regulator regulates a current of a first current path of the differential amplifier based on first signal levels of the input signals. An input enhancement unit steers a majority of the current from the first current path to a second current path based on second signal levels of the input signals.
In yet another aspect, a method includes receiving input signals at a differential amplifier. The method also includes regulating a current of a first current path of the differential amplifier based on first signal levels of the input signals. The method further includes steering current from the first current path to a second current path based on second signal levels of the input signals.